1. Field of the Invention
This invention relates to an antenna duplexer and a transmitting/receiving apparatus using the antenna duplexer, and more particularly, relates to an antenna duplexer and a transmitting/receiving apparatus having an antenna which is alternatively used with a transmission signal and a reception signal the frequencies of which are different from each other.
2. Description of the Related Art
Heretofore, in a time division multiplexed radio communication system, such as a digital car telephone, transmission and reception frequencies are set to be different from each other, and an antenna is alternatively used for transmission and reception by means of an antenna.
For a car telephone, the practical size on an antenna is limited and a terminal unit must be downsized in consideration of the convenience of users. Therefore, it is difficult to use two separate antennas, each exclusively used for transmission and reception, respectively. Thus, one antenna is shared for transmission and reception in the car telephone.
The antenna duplexer is composed of a duplexer circuit or a switching circuit having diodes and transistors.
As shown in FIG. 1, an antenna duplexer 1 sends a transmission signal STX outputted from a transmitting circuit through a band-pass filter (BPF) 2 to a phase shifting circuit 3, an output of which is connected to an antenna 4.
The antenna 4 is further connected to a phase shifting circuit 5 of the receiving side, an output signal of which is delivered to a receiving circuit through a band-pass filter 6.
As shown in FIG. 2, each of the band-pass filters 2 and 6 is composed of a filter with a steep out-of-band damping characteristic in which the frequency bands fT and fR of a transmission signal STX and a reception signal SRX are respectively selected as a pass band so that the reception signal SRX and the transmission signal STX are adequately suppressed.
The antenna duplexer 1 selectively transmits the transmission signal STX output from the transmitting circuit to the antenna 4 and also selectively receives the reception signal SRX picked up by the antenna 4 to the receiving circuit.
The phase shifting circuits 3 and 5 are respectively selected so as to have a predetermined phase characteristic in order to achieve matching with the antenna 4.
However, as shown in FIG. 3, some cases can be considered in which a switching circuit is composed of diodes to thereby share one antenna 4 for transmission and reception.
An antenna duplexer 10 shown in FIG. 3 sends the transmission signal STX to an anode of a diode 12 constituting a high-frequency switch through a capacitor 11 for cutting off a direct current. A cathode of the diode 12 is connected to the antenna 4 through a capacitor 13 for cutting off a direct current.
The antenna 4 is further connected to a cathode of a diode 14 constituting a high-frequency switch through the capacitor 13 for cutting off a direct current. An anode of the diode 14 is connected to a receiving circuit through a capacitor 15 for cutting off a direct current.
The cathodes of the diodes 12 and 14 are grounded in connection with direct current through a choke coil 16 and the anodes of the diodes 12 and 14 are connected to a selector circuit 19 through choke coils 17 and 18.
The selector circuit 19 inputs an output voltage of a DC power supply 20 through a resistance 21 to selectively output the output voltage to the choke coils 17 and 18. Terminals of the choke coils 17 and 18 at the selector circuit 19 are grounded through DC-cutting-off capacitors 22 and 23, respectively.
The diodes 12 and 14 are turned on for transmission and reception, respectively, to selectively connect a transmitting circuit and receiving circuit to the antenna 4.
The antenna duplexer 1 shown in FIG. 1 has a problem that, because the band-pass filters 2 and 6 with a steep out-of-band damping characteristic are generally large in size, the size of the antenna duplexer 1 increases by the dimensions of the band-pass filters 2 and 6 and the insertion loss of the filters increases.
The antenna duplexer 10 having the configuration shown in FIG. 3 has a problem that, because bias current should be supplied to the transmission-side diode 12 during transmission and the reception-side diode 14 during reception, the power consumption increases by a value equivalent to the bias current.
Particularly, a portable telephone has a problem that the power consumption increases by a value equivalent to the bias current even while it is not in use and thereby the operating life decreases.
To solve the problem, as shown in FIG. 4, a method for sharing one antenna by constructing a switching circuit consisting of four transistors (FETs) instead of diodes can be considered.
In an antenna duplexer 35 shown in FIG. 4, the transmission signal STX is sent to the capacitor 13 through a transistor 36 and the capacitor 13 is connected to the capacitor 15 through a transistor 37.
Thereby, the transistors 36 and 37 are turned on during transmission and reception, respectively, to selectively connect the antenna 4 to a transmitting circuit and receiving circuit.
Moreover, the terminals of the capacitors 11 and 15 are grounded at the transistors 36 and 37 with transistors 38 and 39. As a result, the transistors 38 and 39 are turned on during transmission and reception, respectively, and the antenna connection ends of the receiving circuit and transmitting circuit are grounded.
The terminals of the capacitors 11, 13, and 15 are grounded through the transistors 36 and 37 and resistances 40, 41, and 42, respectively, to thereby set bias voltages of the transistors 36 to 39.
The gates of the transistors 36 to 39 are connected to the selector circuit 19 through resistances 43 to 46 and output ends of the selector circuit 19 are selectively grounded through a parallel circuit composed of a resistance 47 and a capacitor 48 or a parallel circuit composed of a resistance 49 and a capacitor 50. As a result, a bias voltage is complimentarily supplied to the transistors 36 and 39, and the transistors 37 and 38, respectively.
The terminals of the switching circuit 19 are switched to turn on the transmission-side transistors 36 and 39 or the reception-side transistors 37 and 38, thereby selectively connecting one antenna 4 to a transmitting circuit or receiving circuit.
However, although the power consumption decreases, this method has a problem that four transistors 36 to 39 with a small high-frequency on-resistance must be used and the transistors 36 to 39 cannot freely be selected.
Alternatively, a method for sharing one antenna by using a line equivalent to 1/4 wavelength of the transmission frequency as shown in FIG. 5 can be also considered.
In antenna duplexer 55 shown in FIG. 5, the transmission signal STX is output to the antenna 4 through a diode 56 and the capacitor 13, and a terminal of the capacitor 13 at the diode 56 is connected to a distributed-constant line 57.
In this case, the distributed-constant line 57 is composed of a line equivalent to 1/4 wavelength of the transmission frequency, an output end of which is grounded at the capacitor 15 in connection with high frequency components so that the output end is equalized with an open end when considering the distributed-constant line 57 from the antenna 4.
The antenna duplexer 55 constitutes a loop circuit for supplying a bias current to the diode 56 with a resistance 58, capacitor 59, choke coil 60, and diode 61 and a positive DC voltage is supplied to a bias terminal TB of the resistance 58 so that the bias current can be supplied to the diode 56.
The antenna duplexer 55 is supplied the transmission signal STX from the capacitor 11 to the antenna 4 when supplying the bias current to the diode 56 to turn on the diode 56. At this time, the diode 61 is turned on to ground a terminal of the capacitor 15 at the distributed-constant line in high frequency.
Therefore, by stopping supply of the bias current to the diode 56, the reception signal SRX received by the antenna 4 is output to the receiving circuit through the distributed-constant line 57.
This system has an advantage that it is unnecessary to make the bias current flow through the diodes 56 and 61 during reception. However, it has a problem that a state equivalent to a case in which an inductance is connected in series by a package of the reception-side diode 61 or the like is obtained and thereby isolation deteriorates between the diodes 56 and 61 and the receiving circuit.
Moreover, there is a problem that since the 1/4-wavelength line is large in size, the whole radio communication device becomes large for the line.
In this case, as shown in FIG. 6, a method for decreasing the overall size by replacing the 1/4-wavelength line with a lumped-constant circuit consisting of capacitors 63 and 64 and a coil is considered. However, this method has a problem that insertion loss increases due to decrease of a selectivity "Q" of a circuit comprising small coils and capacitors compared with a case using a transmission line.
A problem common to cases using a switching circuit is that, even if a diode or transistor is kept turned off, a capacitance between input and output of the diode or transistor cannot be removed completely. As a result, the deterioration of isolation between transmission and reception cannot be avoided and insertion loss increases.
Moreover, when using a switching circuit, a band-pass filter in which a reception band is selected as a pass band must be inserted between the switching circuit and a receiving circuit, like the band pass filter of the antenna duplexer 1. As a result, there is another problem that the insertion loss during reception increases and the sensitivity of reception decreases.